Hi OP! I really empathise. I've been through a serious bout of worry with my middle child - he's 3 in June. His development sounds a lot like your DDs: very limited understanding, only following with gestures, no words, minimal babbling. The HV wouldn't do anything until he was 2 because they said he was too young and his development might just come on.
At 2 he hasn't progressed much and I had the HV come to check him and refer us for everything,,, ASD assessments, hearing, SALT... because I knew the waiting list was so long. Like you, I was also comparing to my eldest who was developmentally always ahead!
anyway long story short, he has come on so much with his development really accelerating over the last 6 months (really between 26 months and 34 months where we are today). It just seems that he's a little late to the party but now his brain is ready to start understanding, talking etc. he's still a bit behind but I'm hopeful that he will catch up over the next 12 months.
Must admit, I completely took it for granted that my eldest DD just picked up everything ... I didn't actually have to teach her anything (that's how it felt). With my son, I really simplify my language, make sure I have his attention, use makaton signing (less so now actually)... and this has all really helped.
i remember reading a thread on here that some kids just seem to bloom between 2 and 6 months and 2 and 10 months, as if a switch just flicks on. That's what I feel has happened with my son.
he's still on all the waiting lists for various types of assessment and support... But I'd rather have him on the lists and have the joy of taking him off if his development catches up, than only adding him to all the waiting lists at a later stage. For you, your little one is a lot younger yet so it's probably too early to get any assessments or nhs support just yet (assuming you are uk).
for what it's worth I also have a younger child, 11 months and her development is advanced too. I haven't done anything different between my middle and youngest and the difference between them is stark ... so be kind to yourself. Just do your best to enjoy the stage she is at now without worrying and over analysing everything.
Perhaps something that might help is a book called 'it takes two to talk'. I found this so helpful in giving me ideas for how to adjust my comms and play style to help my son. It's been a blessing honestly.
sorry for the long waffly post! Xxx